1. Field of the Invention
The present invention relates to a quartz crystal unit with a quartz crystal blank hermetically sealed in a container, and more particularly, to a crystal unit for surface mounting capable of optimally maintaining a vibration characteristic even when a mechanical shock is applied thereto.
2. Description of the Related Art
Since quartz crystal units for surface mounting with a quartz crystal blank hermetically sealed in a container are small and light, such crystal units are incorporated, as frequency and time reference sources, together with an oscillation circuit in portable electronic devices represented by cellular phones. In recent years, there is a demand for crystal units for surface mounting which prevent, when a shock is applied thereto, peeling of a crystal blank inside the crystal units or deterioration of vibration characteristics.
FIG. 1A is a cross-sectional view of a conventional crystal unit for surface mounting and FIG. 1B is a partially enlarged cross-sectional view of the part enclosed with a dotted line in FIG. 1A.
The illustrated crystal unit accommodates crystal blank 2 in container body 1 for surface mounting, puts metal cover 5 on the container body and keeps crystal blank 2 hermetically sealed in the container. Container body 1 is made of, for example, laminated ceramics, has a substantially rectangular plane outer shape, that is, a flat and substantially parallelepiped shape, which looks like a rectangle when seen from above when this crystal unit is mounted on a wiring board. A recess for accommodating crystal blank 2 is formed in a top surface of container body 1. On an inner bottom surface of the recess, there are provided a pair of holding terminals 3 close to the positions of both ends of one side of the inner bottom surface. Metal cover 5 is bonded to the top surface of container body 1 through seam welding or the like to close the recess and in this way crystal blank 2 is hermetically sealed within the recess.
On an outer bottom surface of container body 1, there are provided outer terminals 4 as electrode layers to be used to surface-mount container body 1 on a wiring board. Of these four outer terminals 4, a pair of outer terminals 4 located at both ends of one diagonal of the outer bottom surface of container body 1 are electrically connected to the pair of holding terminals 3 via a conductive path formed in the lamination plane between ceramic layers in container body 1. The remaining two outer terminals 4 are used as grounding terminals. Outer terminals 4 used as the grounding terminals are electrically connected to metal cover 5 via a conductive path (not shown) formed in container body 1.
As shown in FIG. 2, crystal blank 2 is made of, for example, a substantially rectangular AT-cut quartz crystal blank. Excitation electrodes 6a are formed on both principal surfaces thereof such that excitation electrodes 6a are located in oscillation regions of crystal blank 2. Lead-out electrodes 6b extend from the pair of excitation electrodes 6a toward both sides of one end of crystal blank 2. Crystal blank 2 is fixed and held within the recess of container body 1 by fixing these lead-out electrodes 6b to holding terminals 3 at the positions where the pair of lead-out electrodes 6b are led out using, for example, conductive adhesive 7 and is electrically and mechanically connected to container body 1.
Examples of the cross-sectional shape along the longitudinal direction of crystal blank 2 include a bevel shape, convex shape and flat shape. A bevel shaped crystal blank has a thickness which is constant over a certain range of breadth of the central part and decreasing from the central part toward the periphery. A convex shaped crystal blank has a gently varying thickness which becomes a maximum at the center of the crystal blank. A flat shaped crystal blank has a constant thickness over the entire range. When the vibration frequency is approximately 30 MHz or more, crystal blank 2 is formed into a flat shape. On the other hand, when the vibration frequency is lower than 30 MHz, crystal blank 2 is formed into a bevel shape or convex shape through edge dressing to confine vibration energy within the central region of crystal blank 2 and reduce crystal impedance (CI) of crystal blank 2.
In the following explanations, of both ends in the longitudinal direction of crystal blank 2, one end which is fixed to container body 1 by conductive adhesive 7 is called a “first end” and the other end is called a “second end.” Pillow member 8 protruding from the inner bottom surface of container body 1 is provided in the central part in the width direction of the inner bottom surface at the position corresponding to the second end of crystal blank 2. The second end of crystal blank 2 is placed on pillow member 8 without being fixed to pillow member 8. The second end may also be placed above pillow member 8 so as not to contact pillow member 8.
When the cross-sectional shape in the longitudinal direction of crystal blank 2 is assumed to be a bevel shape or convex shape as described above, pillow member 8 is intended to prevent particularly the vibration region in which excitation electrode 6a of crystal blank 2 is formed from contacting the inner bottom surface of container body 1. Also in the case where the cross-sectional shape of crystal blank 2 is a flat shape, the vibration region of crystal blank 2 may also contact the inner bottom surface of container body 1 due to warpage or the like of container body 1, and therefore pillow member 8 is effective in such a case, too.
Such pillow member 8 is provided simultaneously with a tungsten (W) layer or molybdenum (Mo) layer formed as a base electrode layer making up holding terminal 3 using a printing method when ceramic green sheets, i.e., unburned ceramic raw sheets, are laminated and burned to form container body 1. Alternatively, pillow member 8 may be made of ceramics, and burned and formed a part integral with container body 1.
To reliably prevent the bottom surface of crystal blank 2 from contacting the inner bottom surface of the recess, generally, as shown in FIG. 1B, holding terminal 3 and pillow member 8 are formed with an increased thickness by printing two coats of a base electrode layer made of tungsten, molybdenum or the like. That is, as for holding terminals 3, suppose the base electrode layer has a two-layer configuration of first layer 3x and second layer 3y and the sum of the thicknesses of these layers is, for example, 30 μm.
Furthermore, pillow member 8 also has the function of reducing the swinging width of the second end of crystal blank 2 when a mechanical shock is applied to the crystal unit and maintaining the vibration characteristic of crystal blank 2 satisfactorily. Upon receiving a shock, crystal blank 2 swings around the first end as the axis, but the provision of pillow member 8 reduces the swinging width at the second end, and therefore the degree of swinging of crystal blank 2 also decreases and influences on conductive adhesive 7 which holds the crystal blank at the first end also decrease. The vibration system of crystal blank 2 including conductive adhesive 7 has less variation by shock, and can thereby maintain the vibration characteristic satisfactorily and suppress frequency variations.
On the other hand, when pillow member 8 is not provided, the swinging width on the second end side of crystal blank 2 increases when a shock is applied, the influence of the swinging also extends to conductive adhesive 7, causes a variation in the state thereof, that is, the influence reaches the vibration system and deteriorates the vibration characteristic of the crystal blank. In this way, pillow member 8 provided for the second end of crystal blank 2 is meaningful in two aspects; preventing the vibration region of crystal blank 2 from contacting the inner bottom surface of container body 1 and maintaining the vibration characteristic of the vibration system against shocks.
However, the configuration simply provided with pillow member 8 to reduce the swinging width of the second end of crystal blank 2 has limitations in preventing frequency variations due to application of consecutive shocks and aging or the like, or peeling of crystal blank 2. Therefore, as disclosed, for example, in Japanese Patent Laid-Open No. 2004-48384 (JP-A-2004-048384) and Japanese Patent Laid-Open No. 2001-94386 (JP-A-2001-094386), there is a proposal to fix the second end of crystal blank 2 to the container body using an adhesive. In this case, the locations of the second end of crystal blank 2 at which the adhesive can be applied can be both ends or one end of the side of the crystal blank at the second end or central part of such a side. In both cases, the second end of crystal blank 2 is fixed to the container body by the adhesive, preventing swinging thereof in the vertical direction. This eliminates the necessity for providing pillow member 8. However, if fixing the second end of crystal blank 2 using the adhesive eliminates the necessity for pillow member 8, the shared use of container body 1 is not made possible for the case where pillow member 8 is provided without using the adhesive and the case where the adhesive is used but pillow member 8 is not provided, which causes productivity to decrease.
Therefore, as the method of allowing fixing of the second end of crystal blank 2 using adhesive 7 as well as providing a pillow member, as shown, for example, in FIG. 3A, container body 1 with pillow members 8a, 8b provided at both ends in the width direction of the recess at the second end of crystal blank 2 may be used. FIG. 3A is a plan view of a crystal unit with a metal cover removed for convenience of explanation and shows contours of crystal blank 2 with alternate long and short dashed lines. In a case where container body 1 shown in FIG. 3A is used and the second end of crystal blank 2 needs to be fixed, both sides of the second end of crystal blank 2 are fixed using an adhesive. However, in this case, crystal blank 2 is held to container body 1 at four points and stress caused by a difference in thermal expansion coefficient between container body 1 and crystal blank 2 directly applies to crystal blank 2, which deteriorates the frequency-temperature characteristic of crystal blank 2.
On the other hand, when container body 1 shown in FIG. 3A is used and an adhesive is applied only to one of pillow members 8a, 8b to hold crystal blank 2, only one side of crystal blank 2 is fixed at the second end and the adhesive is not applied to the other side, and therefore crystal blank 2 is held by being twisted, which causes the position of crystal blank 2 to be shifted or also affects the state in which crystal blank 2 is held at the first end through conductive adhesive 7.
Alternatively, as shown in FIG. 3B, in container body 1 shown in FIG. 3A, adhesive 9 may be applied onto the inner bottom surface of the recess of container body 1 at the midpoint between two pillow members 8a, 8b and the midpoint of a side at the second end of crystal blank 2 may be fixed to container body 1 using adhesive 9. In this case, as described above, both pillow members 8a, 8b have a two-layer structure of first layer 8x and second layer By and have a large thickness, that is, height, and therefore the adhesive applied onto the inner bottom surface of the recess may not adhere to crystal blank 2. When the amount of adhesive 9 applied is increased, adhesive 9 may adhere to crystal blank 2, but in this case, adhesive 9 spreads in the horizontal direction, adheres to the principal surface of crystal blank 2 more than necessary and deteriorates the vibration characteristic of crystal blank 8. It is difficult to exercise control so that an appropriate amount of adhesive 9 is applied. Applying adhesive 9 onto the inner bottom surface of the recess at a position between pillow members 8a, 8b and fixing crystal blank 2 is not implementable in practical terms.